1. Field of Invention
The present invention relates to a process for the recovery of the uranium contained in phosphated compounds, solid or in solution. More specifically, the present invention is directed to a process for the recovery of the uranium present in an organic phase in the form of a salt of a pyrophosphoric diester.
2. Description of the Prior Art
It is known that the natural phosphates are, essentially, compounds of tricalcic phosphates which absolutely cannot be assimilated by plants, so that they cannot be used as fertilizer. This is why transforming tricalcic natural phosphates into phosphates assimilable by plants and, especially into superphosphates which constitute the widely used fertilizers, represents the heart of the activity of the phosphate industry.
The first stage of this transformation is lixiviation with an acid, usually sulfuric, which converts the tricalcic phosphate into phosphoric acid and into insoluble calcium sulfate. In addition, this lixiviation liberates the different metals such as uranium, thorium, vanadium and the rare earth metals which are trapped in phosphate deposits over the ages.
The concentration of these metals in phosphated ores is very low. Thus, for uranium, it is generally between 0.005 and 0.02%, by weight. Although these concentrations are low, because of the high market value of uranium and because of its strategic characteristics, extensive research has been conducted in the last 25 years to recover the uranium contained in acid lixiviation solutions of phosphated ores.
Among the various recovery processes which have been proposed, a particular one has found important industrial use. This process consists of submitting the lixiviation solution to a liquid-liquid extraction by means of an organic phase containing a diester of pyrophosphoric acid, which will also be called herein dialkyl pyrophosphoric acid.
This technique which is described in detail in U.S. Pat. No. 2,866,680 permits very rapid and very complete removal of the uranium present, even in very low concentrations, in different types of solutions.
However, in spite of the advantages, there are serious drawbacks in this process. The pyrophosphates hydrolyze very rapidly, which necessitates frequent regeneration of the organic phase, thus raising the cost in using this process. Also, the pyrophosphate-uranium complex is so stable that it requires a very powerful reagent to reextract the uranium.
Thus, until now experiments of elution in an alkaline medium have not permitted the regeneration of the pyrophosphoric ester and have generally led to the precipitation of an organic pyrophosphate of uranium which had to be calcined in order to eliminate all organic traces.
Until recently, only the use of hydrofluoric acid gave appreciable results for this reextraction. However, the use of this acid on one hand accelerates considerably the hydrolysis of the pyrophosphates, with all the drawbacks that entail and which have been mentioned above, and on the other hand, requires the use of very costly materials because of the toxic and corrosive characteristics of hydrofluoric acid.
An additional drawback of the above-mentioned process lies in the fact that the diesters of pyrophosphoric acid are not very selective, so that they extract, in addition to uranium, an important part of the other metals trapped in the phosphates. Upon reextraction, these metals are also precipitated by the hydrofluoric acid and they make the uranium fluoride so obtained very impure.
An important step forward in the recovery of the uranium contained in phosphated solutions is described in French patent application No. 77-25899 submitted by the present assignee. Nevertheless, this process requires the application of material which is relatively expensive because of the colloidal nature of the precipitates and the relative difficulty of separating the liquid, aqueous, or organic phases from the solid phases. The colloidal nature of the precipitates is essentially due to their impregnation by compounds of dialkyl pyrophosphoric acid.